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Swelling Pressures, Electric Potentials, and Ion Concentrations: Their Role in Hydraulic and Osmotic Flow Through Clays¹

ABSTRACT

Using mass balance and force balance equations, the mechanisms of convective transport through porous media are outlined.

Pressure differences across compressible porous media cause clay to concentrate at the outflow side until the resulting osmotic pressure gradient balances the hydraulic pressure gradient. The resulting gradient in mobile adsorbed cations causes a diffusion potential which moves solution by electro-osmosis to the outflow side.

When midplane potentials in the pores are appreciable, solution concentration differences across incompressible media are translated into a hydraulic pressure gradient along the midplane of the pores which moves the solution to the high concentration side. Similar concentration differences across compressible porous media result in nonuniform clay distribution, and diffusion potential gradients, which cause electro-osmotic movement of solution to the high concentration side of the media.

These mechanisms provide an explanation for the rapid rates of "osmotic" solution flow observed in response to solute concentration gradients across membranes and other porous media.

¹ Contribution from the US Soils Laboratory, Soil & Water Conservation Research Division, ARS, USDA, Beltsville, Md.; The Negev Institute for Arid Zone research, Beersheva, Israel; and the Dept. of Soil Science & Plant Nutrition, Univ. of Western Australia, Nedlands.

² Director, US Soils Laboratory.

³ Senior Soil Scientist, Negev Institute.

⁴ Professor, Dept. of Soil Science & Plant Nutrition, Univ. of Western Australia.

Received for publication July 16, 1971. Accepted for publication November 19, 1971.